1. Field of the Invention
This invention relates to an X-ray generating apparatus which uses synchrotron radiation (SR) and is advantageously applicable to soft X-ray lithography, for example. More particularly, this invention relates to an X-ray generating apparatus which, by means of an electron undulating ring capable of accumulating electrons as accelerated in an orbit, enables high-intensity X-rays abounding with soft X-rays suitable for fine processing such as transfer of LSI circuit patterns to be generated with high efficiency in a manner suitable for irradiation of a large area
2. Description of the Prior Art
Heretofore, in the production of LSI devices of fine structure, the technique of photolithography has been adopted for the transfer of patterns to resist films.
Owing to light diffraction, the minimum pattern width that can be transferred by photolithography is about 1 .mu.m, a size substantially equal to the wavelength of light. For further reduction of the pattern width, there has been increasingly felt a necessity for a lithographic technique capable of mass transfer of patterns on the submicron order. The technique of X-ray lithography which entails minimal diffraction constitutes one such technique.
As the X-ray source for this technique, an attempt has been made to realize adoption of characteristic X-rays obtainable by bombarding a solid target with an electron beam. The wavelength of an X-ray is inversely proportional to its energy. An X-ray of short wavelength has high energy. In most cases, the X-ray has too high energy to interact strongly with the electrons in a given substance. It simply penetrates the substance without being affected by any local variation in the arrangement of atoms in the substance. In the transfer of a pattern onto a resist film, therefore, the X-ray is absorbed by the resist with poor efficiency and is required to irradiate the resist for a long time. To ensure sufficient mask contrast, the film absorbing the X-ray is required to have a very large thickness. Because of the short wavelength, the photoelectrons generated in the resist film and the substrate have so high energy as to induce diffusion of secondary photoelectrons and degradation of resolving power. To preclude the effects of penumbra and geographic distorsion, an ample distance must be interposed between the X-ray source and the wafer. Since the X-ray source of this type is of a divergent type, the utilization efficiency of the beam decreases in inverse proportion to the square of the distance. To obtain a beam strength that is sufficient from the practical point of view, the X-ray source to be adopted is required to possess very high intensity. Realization of an X-ray source satisfying all these requirements proves infeasible by the existing technical standard.
To solve the problems described above, the soft X-rays of rather long wavelength which are emitted quite abundantly from a synchrotron electron accelerator such as an electron storage ring has come to find acceptance. The lithography making use of the soft X-rays attains density in the transfer of a fine electron circuit pattern at least two orders of ten higher than the density obtainable by the existing method.
The soft X-rays emitted by the existing electron storage ring are such that the intensity and uniformity appropriate for the purpose of transfer of circuit patterns are obtained in a very narrow area on the order of several millimeters in vertical size. As measures for widening the tilt angle amply, there have been devised means of reciprocating the wafer, an article exposed to the X-rays, relative to the X-rays of synchrotron radiation thereby allowing the X-rays to irradiate the entire surface of the wafer uniformly and means of disposing a flat or curved reflecting mirror provided with a swinging mechanism in the path of the X-rays of synchrotron radiation thereby oscillating the X-rays of radiation either vertically or laterally and enabling the X-rays to irradiate the entire surface of the wafer.
When a wafer for the LSI device is lithographically fabricated by the former method which necessitates reciprocation of the wafer itself, however, the positioning of a mask turns out to be a difficult task. Thus, this method is not suitable for mass fabrication of LSIs. Further, it has a disadvantage that it inevitably entails mechanical error. The latter method which involves use of a reflecting mirror suffers from the following disadvantages.
The first disadvange is that since the available spectral intensity of the soft X-rays is variable with the reflectance of the material forming the mirror surface, the exposure time can be estimated only with difficulty. The second disadvantage is that since the reflectance of the mirror surface is gradually degraded by deposition of extraneous substances on the mirror surface due to the irradiation of light, this method requires periodic replacement of the mirror. It also requires the intensity of the soft X-rays to be confirmed each time the irradiation of the X-rays is effected. Further, the degradation of the reflectance does not necessarily proceed uniformly over the entire mirror surface, and the mirror surface, therefore, may cause uneven irradiation of the X-rays upon the wafer.